The AC/Heater is designed to divert to defrost when the system looses vacuum and in most cases its usually a broken and or loose vacuum supply line in the system from the storage reservoir to the firewall. I would start with tracing the supply line from the resivior to the fire making sure its not broken and or come loose. I have also enclosed The Factory test to also help you . Start testing at A1.
This details will help.
Thanks.
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This details will help.
Thanks.
See below latest solved car problems :---